Regeneration of spent pickle liquor and recovery of ferrous sulfate monohydrate



Dec. 15, 1953 J. A. SHAW REGENERATION OF SPENT PICKLE LIQUOR ANDRECOVERY OF FERROUS SULPHATE MONOHYDRATE Filed Nov. 3, 1949 INVENTOR.JOSEPH A. SHAW Patented Dec. 15, 1953 REGENERATION OF SPENT PICKLELIQUOR AND RECOVERY OF FERROUS SULFATE MON OHYDRATE Joseph A. Shaw,Pittsburgh, Pa., assignor to Koppers Company, Inc., Pittsburgh, Pa., acorporation of Delaware Application November 3, 1949, Serial No. 125,365

7 Claims. 1

The present invention relates to the regeneration of spent pickle liquorwhich has been employed in the cleaning or pickling of iron or steel orlike metal articles, and which constitute substantially a mild sulfuricacid solution of ferrous sulfate. c

The enormous quantities of spent pickle liquor which are produced inindustry and the magnitude of the problem of its disposal or utilizationare well known. To meet the problem of disposal of these corrosive andacidic liquors many procedures have been developed for reclamation ofthe waste liquors, the extraction or the iron content, and the recyclingof regenerated liquor. Such processes, although furnishing partialsolutions for this considerable problem, have not swept the industrybecause each of them is subject to one or more disadvantages,particularly one of the following: the costly need for evaporating largequantities of water often under the disadvantageous condition ofprecipitating a difficultly manageable slurry, the necessity OI handlingand circulating sulfuric acid solution of highly corrosiveconcentrations, and the circumstance of precipitating a largeproportion, if not all, of the iron in the waste liquors as copperas,FeSOlflrhO, a salt which can be treated only with dlfiiculty forrecovery of its sullur in the regeneration of the acid for recycling tothe pickling baths.

lt is a primary object of the present invention to provide a process forthe regeneration of waste pickle liquor that is simple and easy tooperate, in which evaporation costs are kept at a minimum, and in whichthe recovered iron is obtained as ferrous sulfate monohydrate from whichsulfuric acid can be readily regenerated.

It is another ob ect of the present invention to.

provide a novel process for regeneration of waste pickle liquors inwhich all of the above-mentioned obJects are obtained and in which, inaudition, the acid concentrations are so regulated that the acidicsolutions can be handled, without; danger of corrosion in commonly usedstainles alloy metals used for handling sulfuric acid solutions.

Another object of the present lIIlVBIltlOIl'lS the provision of aprocess wherein both the free and combined sulfuric acid content ofspent liquor is recovered in sufiiciently' pure form that it can be usednot only in further pickling operation but can also be used in otherassociated processes, in particular, in the ammonia saturator of abyproduct coke plant.

The invention has for further objects such other improvements and suchother operative ad 2 vantages or results a may be found to obtain in theprocesses or apparatus hereinafter described or claimed.

Briefly stated, the present invention comprehends an improved process ofregenerating spent pickle liquor wherein the pickle liquor is, afterremoval from the pickling baths, evaporated to a point at which the salttherein predominantly ferrous sulfate, are just short of the saturationpoint, then adding strong sulfuric acid to the hot concentrated liquorand permitting the mixture to cool to thereby precipitate all but tracesof the ferrous sulfate monohydrate which can be readily slnelted forrecovery of sulfur oxygen gases therefrom. The nitrate from thefiltration of the monohydrate can be concentrated, in whole or in part,and a concentrated portion returned to the regeneration process or allcan be returned directly to the pickling bath or other uses. In case itis observed that some accumulation of extraneous impurities existstherein, the filtrate can be treated Wlth an organic solvent such asbenzene whereby the nitrate is clarified and purirled. In thoseinstances where it is found deslrable to and fresh acid to the picklingprocess, the regenerated acid can be utilized in the time moniasaturator of a by-product coke plant, which plants are usually, forobvious purposes, found closely ad acent pickling operations. Theregenerateo. acid ootalned by the process provides in saturatoroperation an ammonium sulfate oigood quality. it has in the past beenrecognized that it IS advantageous to add SlllIllliC acid to Wastepickle liquor before removal of the salt there- Irom but as suchprevious additions have been perrormed without intermediate evaporationof the solution, the precipitation of large quantities of copperas hasresulted as otherwise in feasibly lar e volullles or acid would havebeen acquired and produced. separation of salt by evaporatlon alone hasresulted also in precipitation of copperas.

it has been found ill the development of the present process that, whilethe character of the ferrous sulrate monohydrate precipitation i primarly governed by the acid strength and the salt concentration of themother liquor, a delay is usually benellclal in removing the last tracesof the salt. lvllnute traces of the salt continue to crystallize out ofthe mother liquor over a period or many days yielding an improvedregenerated acid, probably in accordance with obscure and littleunderstood laws of crystallization. A delay period of three hour will belong enough to produce a recovered acid of-suflicient purity for mostpurposes, but where necessary the acid can be allowed to stand a longertime to achieve more rigid standards of purity.

The evaporation of the waste pickle liquor down to only its saturationconcentration and the combination therewith of subsequent regenerationsteps are so arranged that only about 85% of the water of the wastepickle liquor need be evaporated.

As is well known, the acid strength of waste pickle liquor is low. Afterthe liquor has been concentrated in the present process, a sufficientquantity of strong acid is added thereto that the acid concentration ofthe admixture is immediately raised above the range at which sulfuricacid is most corrosive. The present process employs solutions of acidconcentration below and above about 60%. Consequently, at no time needsolutions of highly corrosive strength be transported in the system, andapparatus embodying the present process do not require the use of themore highly expensive alloys.

In the accompanying drawing forming a part of the specification thesingle figure is a schematic flow diagram of apparatus in which theprocess may be carried out.

In operation according to the present invention and as illustrated bythe aforesaid flow diagram, 1,000 gallons of spent pickle liquor isflowed through valved pipe l0 into a step-wise evaporator I2 whereinwater of approximately 50% to 60% by volume of the pickle liquor isevaporated therefrom. Substantially no precipitation of salts occurs inthe said evaporator 12 because evaporation is not carried beyond theconcentration at which saturation is reached. The approximately 400gallons of concentrate is flowed without substantial cooling through avalved pipe [4 into mixing tank [6 having agitator means I8.Substantially an equal volume of 95% sulfuric acid is added to theconcentrate from a storage tank through valved pipe 22. The concentrate,which has been brought to an acid concentration of 63% to 70% by weight,is flowed from the mixing tank l8 through a valved pipe 24 into acooling and mixing tank 26 having agitator means 28 and cooling coils30. The acidified solution is cooled therein for a period of threehours, during which period the solution is brought to a temperature ofabout C. During said cooling a considerable mass of crystals isprecipitated therefrom. After cooling the so-formed slurry is flowedthrough a valved pipe 32 into a centrifuge 34 by which means salt,constituting ferrous sulfate monohydrate is separated, which saltcontains about 28 gallons of water as water of constitution. The saltalso retains approximately of its own weight of mother liquor.

The monohydrate is carried on the belt conveyor means 36 to storage orto smelting equipment whereby the monohydrate can be readily convertedinto iron oxide suitable for the blast furnace. The iron oxide is ineasily pulverizable form so that it may be also used for many otherpurposes. Liberated sulfur dioxide from said ferrous sulfate smeltingcan be converted by the well known sulfuric acid processes to 100%sulfuric acid, of which in this example 84 gallons can be obtained. Theso-obtained sulfuric acid can be returned to storage tank 20 for use inregeneration of further quantities of spent pickle liquor or it can beotherwise used.

Mother liquor obtained from the separation in centrifuge 34 is flowedfrom a pipe 38 and by operation of valves 4|] in pipe 42, valve 44 inpipe 46 or valve 48 in pipe 50 can be flowed either exclusively orpartly through one or more of three apparatus for its further treatment.Mother liquor flowing through pipe 48 flows into an evaporator 52. Inthe example above described a mother liquor of 630 gallons is obtained.The mother liquor has a sulfuric acid concentration of 63% to 70% andcontained only about 0.040% iron by weight. By evaporation in the saidevaporator 52, 223 gallons of water may be removed to raise the acidstrength to about 95%. The 95% sulfuric acid is pumped through line 54by the pump 56 into the said storage tank 20 wherein it is stored foruse in regenerating further quantities of spent pickle liquor.

If desired, the mother liquor, which contains only about 0.040% iron byweight and has a 63% acid concentration, can be pumped by pump 58through lines 50 and directly to the pickling baths (not shown). Theaccumulation of impurities, such for example, as inhibitors employed inthe pickling process and organic compounds released by the action ofacid on steel, is indicated by the coloring of the mother liquor leavingthe centrifuge 34. Such colored mother liquor is flowed through valvedpipe 42 into the upper end of a scrubbing column 68 wherein the acid isscrubbed with a countercurrent flow of benzol introduced from a storagetank 10 through a pipe 12 into the said column near the bottom thereof.Washed mother liquor settles to the bottom of the column, is drainedthrough a line 14 having a valve I6 into pipe 50, and is pumped by thesaid pump 58 through the line 60 to the pickling baths (not shown).After treatment of the mother liquor, benzol is decanted from the tower68 through a pipe 18 and is pumped through a pipe 80 by pump 82 into acaustic cleaning tank 84 within which the benzol is caused to passthrough a static column of caustic soda solution. At intervals causticsoda solution is renewed from storage 86 through a pipe 88 and usedcaustic solution is drained through pipe 90 by operation of valve 92 andcan be directed to such use, for example, as neutralizing wash waterused after the pickling operation. Washed benzol is pumped from the tank84 through a pipe 94 to the storage tank ID. A second washing tank canbe disposed in parallel to tank 84 for use during replenishment of thelatter tank.

The foregoing procedure provides substantially complete regeneration ofspent pickle liquor so that, aside from make-up for losses, no additionof fresh sulfuric acid is required.

The following examples are presented for the purpose of illustrating thepresent invention.

Example 1 A waste pickle liquor having an analysis as follows:

Sp. Gr. 25/25 C 1.179

FeSOr 143 grams per liter Free H2804 70 grams per liter Ferric sulfate5.3 grams per liter was distilled and approximately 316 parts by weightof water were removed from an original quantity of 699.2 parts by weightof the pickle liquor. 380.3 parts by weight of 95% sulfuric acid wereadded to the hot distillation residue. This mixture was cooled andallowed to stand and thereafter was filtered. The filter cake con-1stituted ferrous sulfate monohydrate plus a small amount of motherliquor. The filtrate contained only 0.074% total iron and constituted69.4% sulfuric acid and therefore was effectively regenerated.

Example 2 Another portion, in this instance 585 parts by weight, of theaforesaid spent pickle liquor, was evaporated to the extent that 325parts by weight of water were removed. 192 parts by weight of 95%sulfuric acid were added to the hot evaporation residue. The mixture wasallowed to cool and was subsequently filtered. The filtrate comprisedregenerated acid containing 58.66% sulfuric acid and contained only0.082% iron. The filter cake consisted of ferrous sulfate monohydrateand a small amount of mother liquor.

Example 3 A spent pickle liquor having the following analysis:

Sp. Gr. 25/25 C 1.237

Ferrous sulfate 230 grams per liter Ferric sulfate 7 .8 grams per literlvInSO i trace H2804 36 grams per liter was evaporated so as to remove121 parts by weight of water from an original sample of 247.4 parts byweight. 118.6 parts by weight of 96.5% sulfuric acid were added to thehot residue. ihe mixture was allowed to stand and was subsequentlyfiltered. The filtrate comprised regenerated sulfuric acid containing62.66% sulfuric acid and only 0.057 total iron. Again the filtered cakeconstituted substantially only ferrous sulfate monohydrate and somemother liquor.

Example 4 A further quantity of the spent pickle liquor employed in thepreceding example was evaporated, so that 122 parts of water wereremoved by distillation from 247.4 parts by weight of the pickle liquor.102 parts by weight of 96.5% sulfuric acid were added. The mixture wascooled and allowed to stand and was then filtered. The filtratecomprised regenerated acid of 59.22% strength and contained only 0.112%total iron. The filter cake was ferrous sulfate monohydrate plus a smallamount of mother liquor.

Example 5 A portion of the waste pickle liquor employed in Example 3 andconstituting 7 41 parts by weight was evaporated so as to remove 368parts by weight of water. 355.8 parts by weight of 96.5% sulfuric acidwere added to the hot residue. The mixture was cooled, allowed to standand then filtered. The filtrate constituted regenerated acid containing63.48% sulfuric acid and only 0.054 total iron. The filter cakecomprised ferrous sulfate monohydrate plus a small amount of motherliquor, the acid of which is, of course, made available for recycling tothe pickling operation when the acid is recovered from the ferroussulfate monohydrate.

The ferrous sulfate monohydrate obtained in the foregoing examples canbe readily roasted for recovery of its sulfur in the form of strongsulfuric acid. The ferrous sulfate slurry obtained in the foregoingexample filtered readily and comprised a heavy grey solid of sandytexture which did not melt upon roasting.

Pickle liquors described above which contain less than 10% concentrationby weight of sulfuric acid or liquors which contain more than 63% andpreferably more than 65% concentration by weight of sulfuric acid may besafely handled in the commonly used stainless steel alloy equipment.Such pickle liquors of maximum and minimum acid strengths respectivelyare substantially non-corrosive to the less expensive stainless alloyssuch as the austentitic stainless steels. The stainless steel whichcontains 18% chromium and 8% nickel is the least expensive of a group ofother austenitic stainless steels which contain more than 18% chromiumand 8% nickel. These latter alloys also generally contain other metals,such as molybdenum and vanadium. The corrosiveness of pickle liquor to18 and 6 stainless steel gradually increases as the acid strengthincreases from 0 to 10% strength, is very corrosive for acid strengthsbetween 10% and 60% sulfuric acid concentration, and decreases rapidlyas the acid strength increases above 65% sulfuric acid.

The corrosiveness of pickle liquors having sulfuric acid strengthsbetween 10% and 60% is less to austenitic stainless steels which containmore than 18% chromium and 8% nickel. A stainless alloy which iscomposed of roughly 88% cast iron and 12% silica (known as Duriron) iscorrosive-resistant to a pickle liquor of substantially any strength ofsulfuric acid. Another stainless alloy composed roughly of one-thirdcopper and two-thirds nickel (known as Monel metal) iscorrosive-resistant to pickle liquor having a sulfuric acid strengthfrom 0 to 70%. The ordinary l8 and 6 stainless steel, Monel metal andDuriron are relatively inexpensive and preferably may be used with thepickle liquor of the present invention.

This application is a continuation in part of my application Serial No.779,108, filed October 10, 1947, for Regeneration of Spent PickleLiquor, now abandoned.

The invention as hereinabove set forth is embodied in particular formand manner but may be variously embodied within the scope of the claimshereinafter made.

I claim:

1. The method of separating into ferrous sulfate and concentratedsulfuric acid pickle liquor that contains less than 10% by weight ofsulfuric acid and has been produced by treating ferrous metal withsulfuric acid, said method comprising in combination the steps ofconcentrating said pickle liquor in the absence of copperas tosubstantially 50% to 60% of its volume by evaporation and withoutsubstantial precipitation of ferrous sulfate monohydrate therefrom;thereafter adding concentrated sulfuric acid to said evaporated liquorto produce an aqueous mixture containing at least 63% to 70% by weightof sulfuric acid; cooling said aqueous mixture to approximately 25 C. toprecipitate substantially all of the iron in the solution as ferroussulfate monohydrate; separating the so-precipitated monohydrate offerrous sulfate from said acid liquor and evaporating water fromso-separated liquor in a vessel of acid-resisting alloy to aconcentration of sulfuric acid suitable for use in pickling liquor forferrous metal.

2. In a method of regenerating spent pickle liquor remaining from thesulfuric acid treatment of ferrous metal so as to recover from saidliquor ferrous sulfate in the form of a monohydrate and to recover fromsaid liquor, in the form of acid substantially free of ferrous sulfate,

the sulfuric acid present in said liquor, the combination of steps whichcomprises: removing by evaporation in the absence of copperas from aquantity of spent liquor of as much water as can be removed withoutprecipitation of solids to thereby reduce the volume of said quantity ofliquor on the order of at least 50%, adding a sufficient quantity ofconcentrated sulfuric acid to the evaporated liquor to precipitate uponcooling the ferrous sulfate therein as an easily filtrable monohydrate,the concentration and quantity of acid added being such as to produce asolution having an acid concentration of 63% to 70% H2804 to therebyprecipitate from said solution substantially all of the ferrous sulfatepresent therein in the presence of sulfuric acid of such high acidconcentration as to be above the range in which it is highly corrosiveto commonly used stainless alloys, separating the ferrous sulfatemonohydrate from the solution by filtration and evaporating at least aportion of the resulting filtrate to remove water in such quantity as toprovide a concentrated sulfuric acid and almost completely free of ironsalts for recycling back to the step of precipitation of the ferroussulfate monohydrate.

3. In a method of regenerating spent pickle liquor remaining from thesulfuric acid treatment of ferrous metal so as to recover from saidliquor ferrous sulfate in the form of a monohydrate and to recoversulfuric acid present in the liquor in a concentration which is nothighly corrosive to commonly used stainless alloys the combination ofsteps which comprises: removal by evaporation in the absence of copperasfrom a quantity of the spent liquor of as much water as can be removedwithout precipitation of ferrous sulfate, adding a substantially equalvolume of concentrated sulfuric acid to the evaporated liquor to producean acid concentration of 63% to 70% sulfuric acid, slowly cooling thesolution to assist in the precipitation of ferrous sulfate monohydrate,filtering the solution to separate ferrous sulfate monohydrate from aregenerated pickle liquor filtrate having an acid concentration above60% sulfuric acid which is not corrosive to commonly used stainlessalloys, evaporating a portion of the filtrate to form concentratedsulfuric acid (at least 95% concentration) free of iron salts, andrecycling the concentrated acid portion back to the process forprecipitating ferrous sulfate monohydrate.

4. In a method of regenerating spent pickle liquor remaining from thesulfuric acid treatment of ferrous metal so as to recover from saidliquor ferrous sulfate in the form of a monohydrate substantially freeof copperas and to recover sulfuric acid present in the liquor in aconcentration which is not highly corrosive to commonly used stainlessalloys the combination of steps which comprises: removal by evaporationat atmospheric pressure from a quantity of the spent 8 liquor of as muchwater as can be removed with-- out precipitation of ferrous sulfate,adding concentrated sulfuric acid to the evaporated liquor to produce anacid concentration of 63% to sulfuric acid, slowly cooling the solutionto approximately 25 C. to assist in the precipitation of ferrous sulfatemonohydrate, filtering the solution to separate ferrous sulfatemonohydrate from a regenerated pickle liquor filtrate having a sulfuricacid concentration above 60% which is not corrosive to commonly usedstainless alloys, evaporating a portion of the filtrate to form con--centrated sulfuric acid (at least concentration) free of iron salts,recycling the concentrated acid portion back to the process forprecipitating ferrous sulfate monohydrate, and washing the regeneratedpickle liquor with benzol to remove organic compound impuritiestherefrom before returning the pickle liquor to a pickling bath.

5. The method of separating into ferrous sulphate and sulphuric acid,pickle liquor that has been produced by treating ferrous metal withsulphuric acid, said method comprising in combination the steps ofevaporating said pickle liquor until the same is saturated with respectto said ferrous sulphate but without any substantial crystallization offerrous sulphate therefrom, thereafter adding sufficient concentratedsulphuric acid to said evaporated liquor to precipitate the iron in thesolution as ferrous sulphate monohydrate and finally separating theso-precipitated monohydrate from said acid liquor.

6. The method of claim 5 wherein sufiioient concentrated sulphuric acidis added to said liquor to produce a solution having an acidity inexcess of about 50%.

'7. The method of claim 6 wherein the evaporated liquor, after theaddition of concentrated sulphuric acid thereto, is cooled to about 25C. to aid in the precipitation of the monohydrate therefrom.

JOSEPH A. SHAW.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 744,795 Reese Nov. 24, 1903 1,045,723 McFetridge Nov. 26, 19121,626,623 Morgan May 3, 1927 1,891,294 Conway et al Dec. 20, 19322,078,088 Mantius et al. Apr. 20, 1937 2,184,419 Fowler Dec. 26, 19392,185,695 Smith Dec. 26, 1939 2,254,788 Ballard Sept. 2, 1941 FOREIGNPATENTS Numb er Country Date 14,665 Great Britain June 17, 1893 of 1892418,994 Great Britain Nov. 5, 1934 623,446 Great Britain May 18, 1949

5. THE METHOD OF SEPARATING INTO FERROUS SULPHATE AND SULPHURIC ACID,PICKLE LIQUOR THAT HAS BEEN PRODUCED BY TREATING FERROUS METAL WITHSULPHURIC ACID, SAID METHOD COMPRISING IN COMBINATION THE STEPS OFEVAPORATING SAID PICKLE LIQUOR UNTIL THE SAME IS SATURATED WITH RESPECTTO SAID FERROUS SULPHATE BUT WITHOUT ANY SUBSTANTIAL CRYSTALLIZATION OFFERROUS SULPHATE THEREFROM, THEREAFTER ADDING SUFFICIENT CONCENTRATEDSULPHURIC ACID TO SAID EVAPORATED LIQUOR TO PRECIPITATE THE IRON IN THESOLUTION AS FERROUS SULPHATE MONOHYDRATE AND FINALLY SEPARATING THESO-PRECIPITATED MONOHYDRATE FROM SAID ACID LIQUOR.